The Leidenfrost effect—the tendency of liquid drops to hover above a hot surface on films of their own vapor—is a seemingly endless source of strange and surprising phenomena. For example, when the surface is etched with a series of parallel sawtooth grooves, the drops spontaneously self-propel (see Physics Today, June 2006, page 17) and can even flow uphill. Other unusual effects arise when the liquid is replaced by a solid or a gel.
Now David Quéré and colleagues at ESPCI Paris have discovered what may be the strangest and most surprising result of all. Rather than looking at a variation on the Leidenfrost theme, they investigated the basic scenario: a liquid drop placed on a flat solid surface. The drop, they found, spontaneously self-propels and rolls like a wheel.
The self-rolling drops have to be just the right size. They must be large enough that their bases are slightly indented when they rest above the hot surface, but not so large that they lose their overall spherical shape and flatten into pancake-like puddles. For water drops, that means having a radius between 0.7 mm and 1.3 mm.
As vapor forms and escapes from between the liquid and the hot surface, it exerts a drag force on the drop’s outer layer, which generates internal fluid flow in the drop. In large, flat drops, that flow takes the form of two or more counterrotating vortices. But a small, spherical drop can’t accommodate more than one vortex, so the whole drop rotates in the same direction.
In principle, the drop could just spin in place like a wheel on an icy road. But the drop’s rotation tips it forward by an angle α, as shown in the diagram. (In reality, α is a few milliradians; it’s exaggerated here for visibility.) The levitation force vector is thus tilted by the same angle, and the drop experiences a net forward acceleration as if it were rolling down a slope of angle α.
In the more than 250 years since Johann Gottlob Leidenfrost first wrote about his eponymous effect, no one ever noticed the self-propulsion on flat surfaces before now. It took a careful experiment to ensure that the drops began at rest and were free from air currents and other confounding forces. (A. Bouillant et al., Nat. Phys., 2018, doi:10.1038/s41567-018-0275-9.)
